1. Field of the Invention
This invention relates to a method for minimizing mercury releases into the environment, and more particularly, this invention relates to a method for removing elemental mercury from a flue gas.
2. Background of the Invention
Emissions of elemental mercury continue to wreak havoc with the environment. Toxicological effects of mercury on humans and animals are well documented, and include neurological disorders, and kidney disorders. The most important route of absorption of elemental mercury is the respiratory tract, probably due to the mono-atomic nature and lipid solubility of mercury vapor. Indeed, deposition and retention of mercury in man is quite high, on the order of 80 percent. (John Doull, MD, et al., Casarett and Doull""s Toxicology 2nd Ed. (Macmillan Publishing Co., Inc., 1980)). The need for mercury removal from effluents is so great that the U.S. EPA has announced that coal-fired power plants will be regulated for mercury emissions by 2004.
Mercury is contained in flue gas, as are a myriad of respiratory system irritants, listed in Table 1 below:
A myriad of methods exist for removing compounds or elements from flue-gas mixtures. The use of activated carbon is particularly noteworthy for its widespread application. Unfortunately, activated carbon injection is expensive, inasmuch as the sorbent has to be changed very frequently in high through-put scenarios. (T. D. Brown, J. Air Waste Manage. Assoc. (1999), 6, 1). Carbon-to-mercury weight ratios of 3000:1 to as high as 100,000:1 have been projected for duct injection of sorbent for mercury control.
Other researchers have demonstrated the photochemical oxidation of mercury by water or hydrogen chloride at room temperature. (H. E. Gunning, Can. J. Chem., (1958), 36, 89 and C. C. McDonald et al, Can J. Chem. (1959), 37, 930.) However, those efforts are relegated to room temperature. Also, injection of water or other compounds are often required.
Attempts have also been made to use ultraviolet light (at 360 nm) to remove elemental mercury from flue gas streams. (P. Biswas, et al J. Air Waste Manage. Assoc., (1998), 48, 113; Environ Eng. Sci, (1998), 15(2), 137). However, these efforts require a titanium oxide catalyst.
U.S. Pat. Nos. 5,863,413 and 6,048,500, awarded to Caren et al. on Jan. 26, 1999 and Apr. 11, 2000 respectively reveal that reactive hydroxyl radicals form when automobile exhaust gases are irradiated with 253.7 nm light. This results in the destruction (oxidation) of carbon monoxide and unburned hydrocarbons in the automobile exhaust. No treatment of mercury-containing or coal-derived flue gas is discussed therein.
Previous attempts to sequester specific compounds or elements from flue gas mixtures often involves first pre-treating the combustion products. For example, in situ conditioning of fly ash via injection of SO3 often is necessary to enhance particulate removal by an electrostatic precipitator. Also, calcium sorbents often are injected into the ductwork of power plants for acid gas removal. And carbon sorbents have been injected in pilot-scale tests for mercury removal from coal-derived flue gas.
A need exists in the art for a method for removing mercury from fluid streams that does not involve high cost or the use of added compounds. The method should accommodate existing emission duct-work. And the method should operate at a myriad of temperatures so as to be applicable for power generation scenarios on an industrial, commercial and residential level. The method also should be applicable to municipal waste site processes.
It is an object of the present invention to provide a method for removing mercury from flue gas that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a method for extracting mercury from fluids. A feature of the invention is that the method can be performed at temperatures of 0xc2x0 F. (or even to just above the melting point of mercury) and at temperatures as high as 350xc2x0 F. An advantage of the invention is that it can be applied to a myriad of effluent scenarios.
Still another object of the present invention is to provide a method for extracting elemental mercury from flue gas. A feature of the invention is the gas-phase oxidation of the mercury via exposure to radiation having a wavelength of approximately 254 nm. An advantage of the invented method is the utilization of a simple ultraviolet irradiation treatment, with simple equipment similar to that used in water treatment plants for the eradication of microbes and oxidation of organic contaminants.
Briefly, the invention provides for a method for removing elemental mercury from a fluid, the method comprising irradiating the gas with light having a wavelength of approximately 254 nm.
Also provided is a method for removing elemental mercury from a fluid, the method comprising placing gas in an enclosure, irradiating the gas with light and removing oxidized mercury from the enclosure.